1. Field of the Invention
The present invention relates to a ceramic heater for use in a gas sensor for detecting a gas concentration, and to a gas sensor including the ceramic heater.
2. Description of the Related Art
As an example, an engine may include a gas sensor for detecting a gas concentration (and an air fuel ratio by extension), the gas sensor being provided with a ceramic heater for activating a sensor element.
As such type of ceramic sensor, one obtained by embedding a heater element made of a refractory metal such as tungsten (W) or molybdenum (Mo) in a ceramic substrate of alumina or the like is used. On the outer face of the ceramic heater, an electrode pad electrically connected to the heater element is provided, and a connection terminal used for externally applying a voltage to the heater element is brazed onto the electrode pad with a braze (see, for example, Patent Document 1).
Since a gas sensor is used in a high-temperature environment or an environment with a widely varying temperature in many cases, it is desired to improve, for example, the heat endurance of a part used for brazing (hereinafter also referred to as a braze part) in the ceramic heater. Regarding this point, the heat endurance of the braze part is improved by using a braze alloy including copper. For example, in Patent Document 1, an alloy of copper (Cu) and gold (Au) is used as the braze alloy. Furthermore, the connection terminal is made of an alloy of a nickel (Ni) base with higher corrosion resistance in many cases as described in Patent Document 1.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-331502-A